We can update the default template and double-check the email message in the company settings to see if the subject lines match, Treas.rgbc.
QuickBooks doesn't auto-change the personalized subject line. To track certain actions, we can check the Audit log feature to determine if the system auto-changes the subject feature.
The possible reason is other users modify the subject line section. To do this, we can check the default template and the settings to verify the same message.
Here's how to check the subject line in the default template:
- Click the Gear icon settings, then select Custom form styles.
- From the Action column of the default template, select the Edit option.
- Select the Email tab, then scroll down and look for the Subject line portion.
- From there, you can change the generated subject line.
- Click Done to confirm.
Once done changing the default template, please know that the email settings in the Company settings from the message section will also updated.
To verify this you can follow the steps below:
- From the Gear icon, choose Account and settings.
- Select the Sales tab and hover to the Message section.
- Click the Pencil icon and check the Email subject line.
- Once you verified the changes, click Save then Done.

Always remember that updating the default template and verifying the email settings will ensure your subject lines stay consistent.
I've added a resource you can browse through on how to receive and process invoice payments when someone pays you in the future: Record invoice payments in QuickBooks Online.
We're always here to address any questions using QuickBooks. Whether it's about managing your invoice templates or navigating automated features. I'm readily available to assist, ensuring your experience is as smooth and efficient as possible.